The invention concerns a liquid transfer device for an analysis unit having a liquid transfer cannula and a capacitive liquid level detector for detecting the dipping of the liquid transfer cannula into an analysis liquid contained in a vessel, wherein the liquid level detector has a signal electrode, a counter electrode and a detection circuit for detecting a change in capacitance between the signal electrode and the counter electrode. The invention also concerns an associated method for controlling the intake of analysis liquid into a liquid transfer cannula and a suitably designed liquid transfer cannula.
In analysis apparatus used for analyzing body fluids, in particular of blood, liquid transfer devices are required in order to transfer analysis liquids, in particular liquid samples or reagents. Common liquid transfer devices are, for example, pipettes which are used for suction samples or reagents out of a first vessel and to expel them into a second vessel as well as dispensers with which the liquid transfer cannula is connected via a hose to a greater stock of the liquid which may be discharged through the cannulas by means of a pump device. Dispensers usually also perform the same function as a pipette.
In association with the present invention, the designation liquid transfer device generally refers to any device facilitating dipping into an analysis liquid in an analysis apparatus to effect any kind of liquid transfer operation (suctioning up and/or expulsion of liquid) using a liquid transfer cannula. The liquid transfer cannula is for example a hollow needle which normally consists of a thin tube made from metal or plastic. For reasons of simplicity this is subsequently also referred to as a cannulau. Other known forms are disposable dosing tips which are thrown away after use and replaced with new ones. They can have a tubular or tapered form, if necessary with a nozzle-type varying cross-section, can be made of metal or plastic, for example a conductive plastic, and are also described in the following as xe2x80x9ccannulaxe2x80x9d.
When the needle is immersed deeply into the analysis liquid, a relatively large amount of excess liquid remains on its outer side. As a result not only can the precision of the dosage be decreased but especially problematic is that the excess liquid contaminates another liquid into which the needle is subsequently submerged (so-called xe2x80x9ccarry-overxe2x80x9d) or an uneconomic large amount of washing agent is required.
In order to be able to better monitor the submersion depth, liquid transfer devices are provided with a sensing device for the detection of the dipping of the cannula into the analysis fluid, usually designated liquid level detectors or LLD. The liquid level detector is connected to the vertical drive used to submerge the cannula into the analysis liquid in order to stop the submersion motion when the tip of the needle has dipped a few millimeters into the analysis liquid. In addition to preventing the problem of carry-over, one must simultaneously ensure that air is not suctioned up which could lead to measurement errors affecting the diagnosis. For this reason, a minimum submersion depth must be maintained, which can be approximately between 0.1 mm and 5 mm.
The vertical position of the cannula simultaneously provides indication of the level of the liquid in the respective vessel. For this reason, the liquid level detector simultaneously facilitates monitoring of the amount of liquid in the respective vessel to issue a signal for example when the supply of a reagent liquid is used up and the reagent bottle must therefore be exchanged.
A conventional principle of construction for the liquid level detector is based on the measurement of the electrical resistance between the cannula and an electrode disposed on the cannula tip. The cannula and the electrode are electrically insulated with respect to each other so that the electrical resistance between them is very high in the dry state. When the cannula and the electrode are submerged, the sample liquid provides a conductive connection so that the electrical resistance changes abruptly. This signal can be reliably detected using simple electronics. This method has the disadvantage, however, that both the cannula and an electrode must dip into the liquid, on which unavoidable amounts of excess liquid necessarily remain. The above mentioned problem with respect to carry-over and associated reduced precision is thereby exacerbated, apart from when disposable dosing tips are used.
In this respect capacitive liquid level detectors are superior. The detection signal for dipping of the cannula into the liquid is thereby given by the change in electrical capacitance between two sensor electrodes via an electronic detection circuit including an alternating voltage source. The first electrode is thereby normally the cannula itself (which is made from metal or from an electrically conducting (metallized) plastic) and is connected to the hot terminal of the alternating voltage source (signal electrode). The counter electrode, which is usually at ground, is disposed on the outer side of the liquid container of the conventional devices (beneath its bottom and partially around the side walls of the container). This electrode is normally part of the container support. When the cannula tip enters into the liquid, the capacitance between the signal electrode and the counter electrode changes due to the electrical conductivity and dielectric properties of the liquid.
These types of liquid level detectors are described in EP-A-0 164 679, U.S. Pat. No. 4,818,492 and EP-A-0 355 791. These publications contain more detailed descriptions, the complete disclosure of which are hereby incorporated by reference.
A basic problem of capacitive liquid level detectors is that the change in capacitance when entering into the fluid is very small compared to other unavoidable capacitances (xe2x80x9cinterfering capacitancesxe2x80x9d such as the connecting cable and the input of the amplifier). The ratio between the useful signal and the interfering signal is therefore poor. A particular problem thereby is that a portion of the interfering capacitance is not constant, but can change as a function of time in a relatively rapid manner. This is particularly true for capacitive interference caused by moving objects (parts of the automated analysis system, hands or other body parts of the person using the apparatus). Particularly in fully automatic analysis apparatus having a plurality of moving components, such interferences are, in practice, unavoidable.
EP-A-0 355 791 addresses a particular problem of this kind (interference by a membrane closing the container) by setting a reference signal when the membrane contacts and during the subsequent downward motion of the needle, detecting the difference relative to this fixed reference signal. This method is directed to the particular application. Interfering capacitances which change between the fixing of the reference signal and the detection of the liquid surface lead to errors in detection.
The liquid level detector described in U.S. Pat. No. 4,818,492 passively compensates for the interfering capacitances of the leads with the assistance of a bridge circuit. Other capacitive interferences are not thereby eliminated, however, and could also lead to improper detection in this particular configuration.
A liquid transfer device for analysis apparatus having a liquid level detector with improved resistance to interference and more reliable operation is known from document EP 0555710 A2. This publication proposes a coaxial electrode configuration including the liquid transfer cannula and having an active shield via a compensation electrode connected to a voltage follower circuit. In addition, in an advantageous improvement thereof, an additional screening electrode functions as a counter electrode at constant potential.
Such a coaxial, in particular tri-axial configuration having active shielding and accompanying reference electrode facilitates, without specific adjustment or adaptation and independent of the constructive details of the surrounding apparatus, the filling amounts and the dielectric properties of the liquid, the detection of the liquid level on all positions in the apparatus which can be reached by the cannula. This is true substantially since the signal path leads from the needle tip, capacitively, to the surface of the liquid and from this point along a conceptual electric conductance along the surface of the liquid and subsequently via a capacitive signal path back to the accompanying reference electrode so that the lower portions of the liquid column have negligible effects. The liquid level detector therefore reacts extremely sensitively to capacitive changes in the vicinity of the tip so that environmental influences do not falsify detection to as great an extent.
It has, however, turned out that the extreme sensitivity in the vicinity of the tip of the liquid transfer needle can also be disadvantageous, since all moist films in the vicinity of the tip are detected as a surface of a compact, firm fluid even when the tip of the needle has not yet reached the actual surface of the liquid. In order to avoid this, special complicated error correction strategies can be developed and applied, such as subsequent displacement, plurality of insertions, pressure measurements or plausibility checks at predictable fill levels.
In particular formation of foam or soap-bubble-like structures can constitute liquid films which can falsify detection of the liquid surface. These structures are relatively long-lived and cannot necessarily be destroyed by penetration of the liquid transfer needle. Such foam layers or soap-bubble-like structures occur, e.g. when shaking a thoroughbred sample, during centrifuge operation of blood samples for the extraction of serum plasma, during transport of reagent rack-packs and by the suspending and stirring of so-called beads coated with streptavidin. These types of foam layers are normally 2 to 5 mm thick. Bubbles formed on the collar of the vessel are also not popped in many cases by the thin liquid transfer cannula.
A liquid transfer device with a capacitive liquid level detector, which solves the problem of recognizing foam by means of an additional temperature-dependent resistance was proposed in document EP 0913671 A1 published on May 6, 1999.
A further problem arising during liquid transfer consists in the fact that through errors arising improper analysis results can be obtained unwittingly if the liquid being transferred is not dosed or not correctly dosed. Reasons for this may not only be foam layers or soap-bubble-like structures on the liquid, but for example also mechanical faults of the cannula, blockages of the cannula or the cannula impinging on the bottom of the vessel.
Mechanical faults of the cannula are critical, especially if disposable dosing tips are used. These are produced by injection molding and are only inspected for faults at random. A complete check of all disposable dosing tips is not possible, so that basically there is a danger that disposable dosing tips with faults such as grooves or holes will be used unwittingly and culminate in incorrect dosing and misleading analysis results.
Blockages, which are also described as clots which can occur during intake of liquid by the cannula, or the impinging of the cannula on the vessel bottom are indeed in principal detectable due to the fact that in the attempt to suction up liquid a high vacuum builds up in the cannula. The extra cost for vacuum measurement apparatus to detect such faults is however relatively high. An especially costly method to control dosing is known in the case of an immunology-analyzer for blood banks, in which every dosing operation is controlled by means of a video camera and image processing.
With capacitive liquid level detectors dosing control has not been known up until now. Basically it would in fact be conceivable to make use of signals available from capacitive level measurement when the liquid is expelled to control dosing. The height and shape of the signals however are very dependent on the individual parameters, for example vessel form, vessel type, fill level and the conductive environment, and are therefore relatively uncertain. Control of the signals during the liquid discharge after multiple pipetting is very difficult and also unreliable on account of the various signal shapes. Also especially critical in the case of capacitive liquid level detection is that this can hardly differentiate between foam and liquid.
Apparatus to analyze liquids in which a detector created from two electrodes is arranged in the fluid channel forming a measurement section to detect air bubbles is known from documents U.S. Pat. No. 5,005,434 and EP 0527059 A1. The measurement section however is arranged in every case in the upper part of the liquid transfer cannula, which has certain disadvantages.
The air bubble detector only indicates correct liquid transfer if the measurement section between the electrodes is filled with liquid. This requires that the dosage volume is greater than the cannula volume so that dosage of small amounts of liquid in the region of a few xcexcl is not possible or only if more liquid is suctioned up than is ejected in the following dosing. The latter has the disadvantage of greater carry-over, less dosing precision and requires laborious washing of the cannula.
A further disadvantage is that even if the air bubble detector has recognized the presence of liquid, undetected air bubbles can exist between the lower end of the cannula tip and the air bubble detector, which leads to incorrect dosing. In addition conventional air bubble detectors are not suitable for use with disposable dosing tips since they must come into contact with the analysis liquid being transferred and therefore negate the freedom from carry-over effect achieved by replacing the disposable dosing tips. In addition no capacitive liquid level controls which are especially difficult due to the generally small change of signal when the immersion of the cannula tip in the liquid is detected are mentioned in the two documents detailed.
From U.S. Pat. No. 5,045,286 a liquid level detector is known in which the immersion of the cannula tip into the liquid is detected by measuring conductivity at the lower end of the tip. This method however is relatively slow and can lead to incorrect dosing when the tip is immersed in foam which forms a conductive film on the outside of the tip since the system does not recognize that no liquid is being suctioned up.
From U.S. Pat. No. 5,855,851 a capacitive liquid level detector is known which has additional conductivity measurement in the vicinity of the cannula tip. Conductivity is measured just above the lower end of the tip on its outside in order to check whether the tip has been submerged too deeply in liquid and must be washed. This known device cannot recognize whether air bubbles have been suctioned into the cannula but only whether undesirable liquid exists on the outside.
The aim of the invention is to design the known capacitive liquid level detectors, especially the triaxially arrangement known from document EP 0555710 A2 with an actively shielded compensation electrode and accompanying screening electrode acting as counter electrode in such a way that they can reliably differentiate between compact solid liquid and foam or liquid films and also in the case of small dosage volumes they can check whether air or liquid has been suctioned up.
The aim in the case of a liquid transfer device of the type described at the beginning with a capacitive liquid level detector is achieved by the fact that the liquid transfer cannula has two control electrodes between which a measurement section is formed, the control electrodes are arranged in such a way that the measurement section primarily passes inside the liquid transfer cannula, at least one of the control electrodes is arranged at a distance above the lower end of the tip of the liquid transfer cannula and the detection circuit comprises a control device which is designed to detect a change in the resistance of the measurement section occurring when the measurement section in the liquid transfer cannula is filled with analysis liquid.
The idea on which the invention is based consists in the fact that to control whether the cannula is immersed in the analysis liquid or to check whether air or liquid has been suctioned up into the liquid transfer cannula, the electrical resistance or correspondingly the electrical conductance of a measurement section which lies in the fluid channel of the liquid transfer cannula and is filled with analysis liquid when functioning properly is detected. A measurement section in this context is therefore a conductive path through analysis liquid which is contained in the cannula. Such a current path can be designed linearly or three dimensionally as a detection zone whereby the beginning and end, i.e. the terminals are the two control electrodes.
Since the electrical conductivity of analysis liquid is higher than that of air, it can be verified in this way whether the measurement section passes through air or through analysis liquid. If in line with the invention the measurement section primarily passes inside the needle and at least one of the control electrodes is above the lower end of the tip of the needle, it will be ensured that the control device can detect the immersion of the needle in foam since the foam cannot lead to greatly increased conductivity of the measurement section. The measurement section can therefore be used to check and verify submersion in a liquid detected by the capacitive liquid level detector and to recognize the intake of air in the cannula.
In principle, even a check of the conductivity of the analysis liquid by means of the measurement section alone, i.e. not in combination with a capacitive liquid level detector, could be used for detection of submersion into the analysis liquid and for recognition of air bubbles. However, such a construction is too slow for most applications to satisfy the demands on detection speed. The combination in accordance with the invention of a quickly responding capacitive liquid level detector along with a slower check by means of the measurement section in line with the invention combines the advantages of both detection possibilities.
The invention enables qualitative control of the liquid transfer with the liquid transfer cannula by monitoring the resistance of the measurement section both when taking up, as also when discharging liquid. With multiple pipetting the individual liquid stages suctioned up in the cannula and divided by separating bubbles can be identified since the separating bubbles change the resistance of the measurement section. A quantitative control of the liquid transfer is possible if the dosing rate with which analysis liquid is taken up or discharged is known and the time lapsing until the resistance of the measurement section changes is measured.
A fault in the cannula, intake of air, blockage of the fluid canal (through so-called clots) or impingement of the dosing cannula on the vessel bottom can be detected because no analysis liquid reaches or fills the measurement section and its resistance does not change. The invention therefore has the advantage that qualitative dosing control is possible whereby it is checked whether analysis liquid has been taken into the cannula or discharged by the latter. In addition quantitative dosing control of the transferred analysis liquid is possible. A further advantage lies in the fact that foam is reliably detected and the surface of the analysis liquid can be detected with certainty so that liquid transfer is always correct. No laborious vacuum measurement is necessary to register blockage of the dosing needle. Therefore the invention achieves aims for which those skilled in the art have been striving for a long time.
In accordance with a preferred additional feature, it is proposed that the liquid transfer cannula, especially the needle tip, forms one of the control electrodes. If the cannula forms one of the control electrodes, the manufacturing expense to produce and if necessary also the technical effort to measure the resistance of the measurement section is reduced. In this case the control electrode can be placed at any point in cannula by corresponding layout of the wires and suitable materials of the cannula. It is especially advantageous if the tip forms a control electrode since it will be ensured as a result that the measurement section passes almost adjacent to the tip.
Another advantageous feature can be that the measurement section is arranged at such a distance from the lower end of the tip that it passes completely through analysis liquid not when the liquid transfer cannula is submerged in the analysis liquid but only when analysis liquid is suctioned up into the liquid transfer cannula. As a result precise control of the liquid transfer by the liquid transfer cannula immersed in the analysis liquid is possible since the measurement section between the control electrodes changes resistance only when analysis liquid is taken up.
It is not necessary for the measurement section to cover the total length of the cannula. It is even desirable, to facilitate control of possibly small dosage volumes, that the measurement section covers a minimum length of the cannula and is placed not too far from its tip.
It may be advantageous if the lower end of the measurement section is arranged above the lower end of the cannula tip. As a result it is possible to slightly immerse the cannula in the analysis liquid without the lower control electrode being wetted or the measurement section already reacting to analysis liquid so that any air bubbles taken up can be reliably detected. The lower end of the measurement section is preferably between 0.5 mm and 5 mm above the lower end of the tip.
In order to be able to reliably control also small volumes of analysis liquid it is advantageous if the measurement section is adjacent to the tip so that not much analysis liquid has to be taken up into the liquid transfer cannula before a change in the resistance of the measurement section occurs. According to a further advantageous feature it is therefore proposed that the upper end of the measurement section is adjacent to the tip preferably between 0.5 mm and 30 mm above the lower end of the tip.
Additional conductivity measurement of a measurement section in combination with a capacitive liquid level detector in line with the invention is in principle advantageous with all capacitive liquid level detectors, independent of whether the capacitance of the liquid transfer cannula is measured relative to ground or whether the liquid transfer cannula is part of a coaxial electrode configuration. In general control with a measurement section is always advantageous if the capacitive liquid level detector is configured so that it is extremely sensitive to capacitance changes in its surroundings (samples, rotor, reagent vessels, static charges etc.) and in particular when it is extremely sensitive to capacitance changes around the tip of the liquid transfer cannula. On the contrary control of the measurement section will not have any substantial special advantages in recognizing foam if the mass of the detected liquid itself is incorporated into the signal path since in this case the foam or bubble formation does not substantially affect detection of the liquid surface.
The invention is therefore preferred with coaxial electrode configuration in accordance with document EP 0555710 A2 to which full reference is made in this respect i.e. in the case of coaxial electrode configurations which advantageously have active shielding via a compensation electrode connected to a voltage follower circuit and/or a screening electrode as counter electrode and extending into the region of the tip of the liquid transfer cannula.
A first preferred additional feature can therefore consist in the fact that the liquid transfer cannula is part of a coaxial electrode configuration which has, in addition to the liquid transfer cannula, at least one coaxial electrode surrounding same and insulated therefrom. An additional advantageous design feature consists in the fact that the coaxial electrode configuration has a screening electrode surrounding the signal electrode, wherein the screening electrode lies at a constant potential and acts as counter electrode.
Another advantageous feature can be that the detection circuit has an AC voltage source and a voltage follower circuit and the input and output of the voltage follower circuit are connected to two neighboring electrodes of the coaxial electrode configuration constituting signal electrode and compensation electrode so that there is no voltage difference between the signal electrode and compensation electrode and the capacitance between the signal electrode and the compensation electrode is compensated. In accordance with a further advantageous feature it can be possible that a first electrode of the coaxial electrode configuration is the signal electrode of the liquid level detector and is connected with the input of the voltage follower circuit and a second electrode of the coaxial electrode configuration adjacent to the signal electrode is connected with the output of the voltage follower circuit.
Advantageously it is also possible that the liquid transfer cannula as signal electrode is connected with the input of the voltage follower circuit and the adjacent coaxial electrode as compensation electrode with the output of the voltage follower circuit.